This invention relates to a diffractive optical element, an optical system or an exposure apparatus having such diffractive optical element, and a device manufacturing method using such exposure apparatus.
Semiconductor device manufacturing technology has advanced remarkably, as have microprocessing techniques. Recently, reduction projection exposure apparatuses (xe2x80x9csteppersxe2x80x9d) having a resolving power on the submicron order are used prevalently in microprocessing works. Much research has been done to further improve resolving power. For example, the numerical aperture (NA) of projection optical systems have been enlarged, exposure wavelength has been reduced (narrowed), and novel optical elements, such as diffractive optical elements, have been used. Diffractive optical elements are optical elements having a plurality of predetermined concentric and step-like patterns formed on their surface. The patterns diffract light, incident thereon, at a desired deflection angle. These patterns are very thin, and occupy a very small space, and correct well for chromatic aberration.
A diffractive optical element can be formed with an ideal shape (blazed shape) if the steps (surface level differences) of the step-like pattern are formed very small. As such, a very fine structure may be formed. For example, in order to provide a step-like pattern for a four-level structure, the patterning operation must be done twice. In order to provide a step-like pattern for an eight-level structure, the patterning operation must be done three times. Particularly, in order to produce a step-like structure for eight or more levels, an etching operation must be done to a substrate with a relatively large surface step or steps. With such a substrate, it is very difficult to form each step into an exact, predetermined rectangular shape. Furthermore, there is a high probability that the desired rectangular shape will be flawed. An example of such a flaw would be an edge that is undesirably rounded. Such an error in the desired step-like pattern shape will cause a decrease of diffraction efficiency and an increase of unwanted diffraction light. Such errors result in degradation of the optical performance.
It is an object of the present invention to provide a diffractive optical element by which degradation of the optical performance such as described above can be prevented or reduced effectively.
It is another object of the present invention to provide an optical system or an exposure apparatus having such a diffractive optical element.
It is a further object of the present invention to provide a device manufacturing method using such an exposure apparatus.
In accordance with an aspect of the present invention, there is provided a diffractive optical element, comprising a plurality of step-like patterns each having surface levels of a number less than eight, said step-like patterns being disposed in combination to provide a function of a step-like structure of eight or more levels.
In one preferred form of this aspect of the present invention, two of said plurality of step-like patterns are disposed parallel to and opposed to each other.
In one preferred form of this aspect of the present invention, each of the steps of said plurality of step-like patterns have a width of 1 micron or more, wherein each of the steps of said step-like structure of eight or more levels have a width of 1 micron or less, and said step-like structure as a whole can serve as a diffractive optical element for diffracting light, incident thereon, at a desired deflection or diffraction angle.
In one preferred form of this aspect of the present invention, said step-like structure can serve as a phase type computer hologram for forming a desired light pattern.
In one preferred form of this aspect of the present invention, a clearance is formed between the step-like patterns opposed to each other, and the clearance is held in a vacuum state or is sealingly filled with a predetermined fluid.
In accordance with another aspect of the present invention, there is provided an optical system, comprising: a diffractive optical element as described above; and at least one lens disposed in front of or at a back of said diffractive optical element.
In accordance with a further aspect of the present invention, there is provided a projection exposure apparatus, comprising: a light source for producing illumination light; a first optical system for projecting the illumination light to a reticle, and a second optical system for projecting a pattern of the reticle onto a substrate, with light passed through the reticle; wherein at least one of said first and second optical systems includes a diffractive optical element as described above.
In accordance with a yet further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: applying a photosensitive material to a substrate; exposing the substrate coated with the photosensitive substrate, with a device pattern by use of an exposure apparatus as described above; and developing the exposed substrate.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.